Surfaces of workpieces, especially metallic workpieces, to be coated with paint, synthetic resins or other coating materials must be thoroughly cleaned in order to provide a satisfactory bonding between the coating and the substrate surface. While a particularly thorough cleaning, accompanied by grease removal, rust removal and descaling, can be effected by sand blasting, the entrainment of corundum or other abrasive particles in a gas stream against the surface, or the propulsion of steel grit at high speed and kinetic energy thereagainst, the conventional processes are not always satisfactory.
In most cases, the abrasive particles are projected against the surfaces to be cleaned by dry blasts of compressed air in cabins with the blasting turned on and off by a pedal-operated valve while a changeover from one abrasive to another can be effected by the use of a control handle connecting one or another dispenser or hopper containing the several abrasives to the sand-blasting gun. Such systems are suitable for the cleaning of large parts and are not always applicable for the cleaning of small-diameter or small-cross-section continuous members such as rods, wires, tubing, pipe and similar structures.
In another conventional process, the abrasive particles receive their kinetic energy by being cast against the workpiece with blade wheels rotating at high speed. Such wheels can throw 100 to 200 kg of abrasive per minute at a velocity of 70 to 80 meters per second against the surfaces to be cleaned (see Metalloberflache 23, 1969, pages 262-265).
For rapid descaling of tubes, a system is described in Metalloberflache 22, 1968, pages 344 and 345, which comprises an evacuated head chamber and a head connected to an abrasive hopper. The abrasive flows uniformly out of the hopper by the action of the vacuum or suction produced in the head and is carried by compressed air through a plurality of nozzles into the chambers of the surface-treating unit.
In another process which has been employed for rapid cleaning of workpieces, a fluidized bed of sand is maintained by passing compressed air therethrough and residues upon the workpiece such as oils and greases, are volatilized at temperatures up to 450.degree. C. The temperature of the bed can be held constant and the process operated automatically as described in Metalloberflache 30, 1976, page 495.
While all of these processes have been satisfactory to a greater or lesser extent for specific workpieces, they have not been found to be totally successful for the treatment of continuous workpieces for various reasons. For example, the prior-art processes can give surfaces which are sufficiently clean, i.e. grease and scale are removed and rust is eliminated to the extent required by German Industrial Standard (DIN) 18,364 degree No. 3.
However, the roughening of the surface is limited at best and these processes have been found to provide only a moderate increase in the surface area as is required for effective bonding.
The conventional processes also have the disadvantage that they generate high noise amplitudes, create dust and occasionally release noxious or toxic materials into the environment. As a consequence, they are ecologically disadvantageous.
Furthermore, in the conventional systems, all parts contacted by the abrasive are subject to extremely heavy wear so that they are continuously destroyed, requiring maintenance and replacement of the hoses and blasting nozzles or the impellers of centrifugal apparatus in an overly frequent manner.
Those processes and apparatus which require a vacuum or an elevated temperature to be used have the further disadvantage that they must be continuously and precisely controlled, the control systems and personnel involving high cost.